Methods for producing plastic lenses of various prescriptions are well known. For example, Applicant's U.S. Pat. No. 4,873,029 discloses a method for making plastic lenses of ophthalmic quality for eye glasses. Additionally, U.S. Pat. No. 3,248,460 discloses a method for casting plastic lenses from thermosetting or thermoplastic materials. According to this method, a plastic blank having significantly less curvature than is required for the final intended prescription of the lens is used as a base onto which is cast an additional layer of material. The U.S. Pat. No. 3,248,460 employs a conventional optical gasket to provide a space or cavity between the plastic blank and the mold and to hold the resin material in the cavity. The additional layer of material alters the curvature of the resulting lens over the vast majority of its surface, hence changing the prescription of the resulting finished lens to the required power. Another method of making a plastic lens is disclosed in U.S. Pat. No. 3,946,982, in which a conventional optical gasket is used to cast an entire lens surface with a prescription layer.
Conventional industrial lens casting techniques require the use of "conventional optical gaskets" which hold together the components used to cast a semifinished lens which is subsequently ground and polished to the desired prescription. In most cases these conventional optical gaskets can only be used once before they must be reprocessed. Therefore, a significant number of different gaskets must be maintained in inventory.
A one-step process for casting finished multifocal lenses is even more demanding with regard to the required number of different conventional optical gaskets and the inventory of gaskets necessary to produce a variety of different finished lens multifocal prescriptions. In one such system, approximately 737 conventional optical gaskets must be maintained in inventory to allow the production of the full range of prescriptions. Moreover, these gaskets must be constantly replaced after only one use. In addition to the gaskets, approximately 200 "optical center movers" (OCMs) must also be inventoried to relocate or decenter the optical center. These OCMs are also not reusable and, hence must be constantly replaced.
Methods for fabricating multifocal lenses by casting a thin layer of resin onto a preformed lens are disclosed in U.S. Pat. Nos. 4,623,496 and 4,190,621. The '496 patent does not require the use of a gasket, while the '621 patent employs edge fixtures to maintain a predetermined separation between the preformed lens and the mold which is needed to form a resin layer of requisite thickness. Other known methods attempt to manufacture multifocal or progressive plastic lens using a lamination technique. Such a technique joins a preformed plastic section to another cured plastic prescription lens. A portion of the preformed section, which defines a multifocal or progressive region of the finished lens, is joined to the prescription lens with adhesive.
Another method of manufacturing a multifocal lens from a preformed lens is described in Applicant's U.S. Pat. No. 5,178,800, which describes a process of casting a resin layer over a preformed lens to form a multifocal lens without changing the lens correction at the optical center. U.S. Pat. No. 5,219,497 describes a photochemical casting process for manufacturing multifocal ophthalmic lenses from a preformed lens in which the lens correction either remains the same at the optical center or is changed in a predetermined manner, depending on the thickness of the resin layer applied.
In all cases, a key objective is to reduce the center and edge thickness of plastic progressive addition lenses, since the lenses are thereby rendered lighter, and cosmetically more attractive. Industrial lens casting processes seek to achieve reduced thickness by applying prism thinning techniques when semifinished lenses are ground and polished. For the most part, casting whole progressive addition lenses or casting a new progressive addition optic onto a preformed lens do not include the subsequent grinding of the lens optic, and hence cannot be thinned by prism thinning surfacing techniques. Therefore, there is a need for a method to directly cast a progressive addition optic onto a preformed lens which minimizes the center and edge thickness of the resultant progressive lens.